Beam collector



Feb. 24, 1959 A. M. SERANG 2,875,362

BEAM COLLECTOR Filed Oct. 31, 1956 1 INVENTOR ABDUL GAFOOR M. .SERANG BY W ATTORNEY United States PatentO BEAM COLLECTOR Abdulgafoor M. Serang, Menlo Park, Calif., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts Application October 31, 1956, Serial No. 619,541 1 Claim. (Cl. 313-89) This invention relates to high frequency amplifiers of the traveling wave type and has for its object the im provement of beam collectors for such tubes.

A type of traveling wave tube now in common use generally comprises an elongated envelope enclosing a helix, electrodes to direct an electron beam axially through the helix, a collector for intercepting the electron beam after its passage through the helix, and input and output coupling means for the high frequency signal. As is Well known, when the helix is excited the high frequency wave travels along the helix and there is mutual interaction between the electron beam and the high frequency field of the helix. In the usual case, the collector for the beam is of a cup-shape and formed of conducting material with the consequence that electrons impinging thereon cause secondary electron emission from the collector. Such secondary electrons cause undesirable effects in the performance of the tube, in that secondary electrons traveling in a direction opposite to the main electron beam increase the noise figure of the tube and tend to make the amplifier unstable. Consequently, it is desirable to prevent the production of secondary electrons and to inhibit their travel toward the cathode of the tube. Heretofore, secondary emission has been minimized by making the collector in the form of a deep cylindrical cup and coating theinner surfaces of the cup with soot. In small diameter traveling wave tubes, however, the inside diameter of the collector cup is necessarily so small that it is difficult, if not impossible, to satisfactorily deposit the soot on the inner surfaces of the cup, and unless the coating is relatively uniform the advantages of the cup-shape are not realized.

In accordance with this invention, the collector is of a construction to offer the characteristics of a deep cylindrical cup, yet permits soot to be conveniently and uniformly deposited on its internal surfaces. More specifically, the collector is formed in two parts, a cylindrical plug having one end inclined at an angle to the axis of the electron beam to provide the bottom for the cup, and a tightly wound helical spring surrounding the plug and providing the cylindrical walls of the cup. The pitch of the spring is approximately equal to the diameter of the wire of which the spring is wound and consequently forms, for electrical purposes, when contracted, a cylindrical conducting wall. The inner surfaces of the cup are coated with soot, which may be easily deposited by stretching the spring and holding it over a sooty flame. The carbon from the flame passes through the spaces be tween turns of the extended spring and thoroughly coats the spring, inside and out, and when the spring is released to its original shape, the resulting cup is uniformly coated on the inside.

A better understanding of the invention and its construction will be had from the following detailed description when taken with the accompanying drawing in which:

Fig. 1 is an elevation view, partially in section, of a typical traveling wave tube in which the beam collector of the invention may find application; and

2,875,362- Patented Feb. 24, 1959 Fig. 2 is an enlarged elevation view, partially in section, of the beam collector of the invention.

In Fig. 1 there is illustrated a typical traveling wave tube 16 generally comprising a glass envelope enclosing a source of electrons, such as electron gun 18, at the enlarged left end portion 14, and a helix 20 in the reduced diameter portion 12. The electron gun 18 is arranged to direct a beam of electrons along the axis of helix 20 for collection at the opposite end of the tube by collector 22. An electromagnet (not shown) customarily surrounds the envelope in the vicinity of helix 20 to provide a longitudinal magnetic focusing field to confine the electron beam and enable it to pass completely through the helix 20. Means for coupling the signal to be amplified to the helix 2!! andthe output coupling means for the amplified signal have been omitted from the drawing for clarity, inasmuch as they are not necessary for an understanding of the present invention. Appropriate operating potentials are applied to the electrodes of the tube by suitable voltage sources 30, 32 and 34.

The operation of tubes of this type is well known and a discussion thereof is unnecessary for the understanding of the present invention. Insofar as the present invention is concerned, sufiice it to say that electrons from electron gun 18 after acceleration through the tube under the in fluence of the accelerating potential 34 cause deleterious emission of secondary emission of secondary electrons from the metal of which collector 22 is formed. These secondary electrons tend to travel toward the opposite end of the tube, increasing the noise figure of the tube and contributing to instability of the amplifier. In accordance with this invention, the collector is of a construction to inhibit secondary electron emission and to limit the escape from the collector of such secondary electrons as may be emitted.

Referring to Fig. 2, the improved beam collector consists essentially of two parts, a cylindrical plug 46 having one end inclined at an angle to the axis of the cylinder, and a tightly wound spring 42, tightly enclosing plug 40. Plug 40 may be formed of any suitable conducting material, tungsten being preferred because of its high temperature characteristics, and is provided at one end with a stem 46 for mounting the collector on the glass envelope of the traveling Wave tube. The stem 46 may also be formed of tungsten, or other material to which glass can be sealed, and is brazed or otherwise suitably joined to the plug 40. Spring 42 is formed of fine wire and wound to have a pitch approximately equal to the wire diameter so as to constitute, in its unstretched con dition, a short metal cylinder. With the spring assembled over the plug 40 as shown, together they form a cup-shaped structure having a cylindrical Wall and an inclined bottom. It can be shown that the distribution of secondary electrons from a plane surface is essentially a cosine function of the angle at which primary electrons impinge, and accordingly the angle of incline of the plug 40 is selected such that substantially all of the electrons emitted from the inclined surface are intercepted by the cylindrical wall of the cup.

To inhibit the emission of secondary electrons, the inner walls of the cup may be coated with carbon, for example, soot, or the like, to prevent the electrons of the beam from actually impinging upon the metal of the collector. Normally the small diameter of the cellector, shown greatly enlarged in Fig. 2, makes the task of satisfactorily coating the inner walls difiicult, but the present construction conveniently overcomes this problem. To deposit a coating of soot on the inner wall of the cup, spring 42 is extended to provide an appreciable spaced between turns, and the assembly placed in a sooty flame, for example a kerosene flame, until a coating of a thickness sufiicient that the metal surfaces of the Wire and the plug cannot be seen through the coating is deposited. After the soot is thus deposited, the spring is slowly released to its unstretched condition, the cylindrical wall provided by the spring and the inclined surface oflplug 40 being uniformly covered with a thin coating 44 of carbon. Some soot, of course, is also deposited on the outside of the coil, and while it normally will not alfect the eifectiveness of the collector or the operation of the tube, it may be wiped ofi, if desired, without disturbing the coating on the inside walls. After fabrication of the collector as just described, it is sealed at the end of the enevolpe 12 of the traveling wave tube in the usual manner.

From the foregoing it is seen that applicant has provided a beam collector which permits the inner walls of the small diameter cylinder to be uniformly coated with soot to minimize the emission of secondary electrons from the collector. While the collector has been described in connection with a traveling wave tube, it will be understood that it may also have utility in other devices requiring the collection of an electron beam. Preferred materials for the construction of the collector have been suggested, it'being understood, however, that any materials suitable for a particular application may be used without departing from the spirit and scope of the invention. The foregoing description, therefore, is not to be construed as limiting, but merely illustrative of'one' form the invention may take.

1 What is claimed is: 'In an electron beam device, a beam collector positioned in said device to intercept an electron beam and comprising an extensible helical spring formed of small diameter wire and having a pitch substantially equal to the wire diameter, a cylindrical plug having a diameter so that adjacent coils are contiguous when the spring is in relaxed condition and thereby form a hollow tube substantially equal to the inner diameter of said helical spring and a length shorter than said' spring fitted within one end of said spring, the end of said plug within said spring being inclined to the axis of said electron beam at an angle such that substantially all secondary electrons emitted therefrom upon impingement of electrons in said beam are intercepted by the inner surfaces of said spring, and a substantially uniform coating of carbon on the inclined surface of said plug and the inner surfaces of said spring.

References Cited in the file of this patent UNITED STATES PATENTS 1,865,449 Wuertz July 5, 1932 1,961,748 Ewest June 5, 1934 1,989,954 Van Gessel Feb. 5, 1935 2,107,520 Schade Feb. 8, 1938 2,280,980 Samuel Apr. 28, 1942 2,295,680 Mouromtseff et a1 Sept. 15, 1942 2,431,135 Morse Nov. 18, 1947 2,477,279 Anderson July 26, 1949 2,611,335 Dilts Sept. 23, 1952 2,619,706 Vause Dec. 2, 1952 2,719,936 Peter Oct. 4, 1955 2,727,179 Lally et al. Dec. 13, 1955 line 4, same column.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,875,362 February 24, 1959 Abdul'gafoorM. Serang ppears in the printed specification equiring correction and that the sai It is hereby certified that error a of the above numbered patent r d Letters Patent should read as corrected below. I Column 4, lines 5 and 6, strike out "so that adjecenfcoilsww contiguous when the spring is in relaxed on n d t h rebyiorm a hollow tube" and insert the same after "diameter", first occurrence, in

Signed and sealed this 7th day of July 1959 E L) f Attest:

KARL H. AXLINE ROBERT c. WATSON Attesting; Officer Comriissioner of Patents 

